It has become common practice to use light emitting diode (LED) displays for a variety of purposes. Typically such displays are manufactured as seven-segment displays or alphanumeric displays, and, if desired, can be arranged as dot matrix displays. Such displays require multi-color and high brightness and must have a thin profile.
In a typical manufacturing process custom display devices use the concept of stretching the light from an LED by diffusion and reflection. The LED chips are mechanically attached onto a printed circuit board (PCB) or lead-frame by using electrically conductive adhesive, e.g. silver epoxy. Gold (or other conductive material, such as aluminum) is used to wire bond the top of the LED die to the PCB. A cone shaped reflecting cavity is cast inside a rectangular package around each LED. A plastic housing, often referred as ‘scrambler’, forms the display package and contains the LED segment cavities. The housing also provides structural integrity to the LED package. Generally, the material used for the scrambler is polycarbonate with TiO2 sealant to prevent light leakage. Optical grade epoxy fills the top of the cavity and also fills the bottom of the scrambler to form the stretched segment.
Presently, these custom LED display packages are predominately through-hole mounted because of economy of manufacture. However, surface mounting assemblies are quickly replacing wave-soldering techniques because wave soldering has reached the limit of its capabilities. Currently, reflow soldering has become the leading technique for soldering components, such as LED packages to PCBs. Miniaturization of control panels and simplified manufacturing processes are requiring LED manufacturers to convert through-hole devices to surface-mountable devices.
One manufacturing process now being used for surface mounting LED packages to PCBs is a lead-frame process where a metal frame is folded around a substrate holding the LED. Such processes are time-consuming and cumbersome. An alternate process for surface mounting is to mount the LED onto a PCB for support purposes and to then surface mount the PCB onto a controller PCB board. Because of surface irregularities between the two PCBs, such PCB/PCB mounting is difficult to achieve in a reliable manner.
In general, the PCB to PCB or even lead-frame to PCB mating tends to face surface irregularities as a result of PCB warping or lead-frame lead coplanarity issues. In addition, as the customized displays become larger and have more LED segments, the warping becomes more pronounced and adds further complexity to the soldering process.